1. Field of the Invention
The present invention relates to a method for controlling the air-fuel ratio in an internal combustion engine. The method according to the present invention is applicable to an automobile engine.
2. Description of the Prior Art
One known type of apparatus for controlling the air-fuel ratio in an internal combustion engine includes means for generating a fundamental fuel signal representing engine fuel demand in a steady state of the engine in correspondence with values of predetermined engine operation parameters, including engine temperature; means for detecting a transient operation state of the engine representing output power increase demand; means, responsive to the measured engine temperature and the detected transient state of the engine, for generating a reinforce promotion signal which has an initial value determined by the detected transient state of the engine and which is increased by a factor changing toward unity at a rate decided by the measured engine temperature; and means for supplying fuel to the engine in accordance with the fundamental fuel signal and the reinforce promotion signal so as to supply the engine with fuel in accordance with the fuel demand. This type of apparatus enables a fuel supply system with a constantly optimum air-fuel ratio not only in a steady state but also in a transient state of the engine and thus enables constantly optimal engine operation. Such an apparatus is disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 56-6034.
In this type of apparatus, however, no consideration is given to long-term changes in the operating characteristics of the engine, for example, changes in characteristics due to deposition of a viscous material such as fine carbon particles originating from lubricant constituents and combustion products at the valve clearance or at an injection nozzle of an electronic fuel injector and changes in characteristics due to such deposition at the rear surface of a cylinder intake valve.
Clogging of injectors may be compensated for by a feedback operation by an air-fuel ratio sensor in the case of steady-state operation, but this has not been possible in transient-state operation due to the absence of correction means. Also, this type of apparatus does not take into consideration inevitable variations in and aging of the structures of the manufactured engines or airflow meters.
Further, it does not consider the problem of the seasonal difference in specific properties of the gasoline used. Usually, a gasoline producer sells different kinds of gasoline for each season of the year. These, of course, differ in volatility characteristics, as expressed by Reid vapor pressure or distillation characteristics. Even gasolines from the same producer vary from 0.5 kg/cm.sup.2 to 0.86 kg/cm.sup.2 in vapor pressure or from 40.degree. C. to 58.degree. C. in 10% recovered temperature.
Such differences in volatility characteristics result in considerably different air-fuel characteristics in the transient operation state.
When engine operation characteristics change due to long-term deposits or when low volatility gasoline is used, the air-fuel ratio in the acceleration state becomes relatively lean. Hence, the engine operation deteriorates, e.g., non-smooth acceleration occurs. On the other hand, the air-fuel ratio in the deceleration state becomes relatively rich. Hence, emission and the specific fuel consumption deteriorate. Even when a high volatility gasoline is used, the air-fuel ratio becomes rich in the acceleration state, resulting in the same problems.
A technique for the control of the air-fuel ratio to overcome the above problems has been proposed in Japanese Patent Application No. 58-3288 (corresponding to U.S. Ser. No. 566,815), however, this still requires further improvement.